Beacon signaling in a wireless system

ABSTRACT

A few high power tones used for synchronization and/or other purposes are transmitted in a FDM system during a period of time, e.g., a symbol transmission time period. During normal data transmission symbol periods signals are transmitted using at least 10 tones, e.g., per symbol time. Less than 5 high power signals are transmitted in a symbol time with at least 80% the maximum total transmitter power transmitted being allocated to the high power signals where the maximum total transmitter power is determined from a period of time which may includes one or more data and/or high power tone transmission periods. When the high power tones are transmitted at most 20% of transmitter power is available for transmitting other tones with the power normally being distributed among multiple tones. Normally some tones which would be transmitted in a symbol time go unused during transmission of the high power signals.

RELATED APPLICATIONS

[0001] The present invention claims the benefit of U.S. ProvisionalPatent No. 60/406,076 filed Aug. 26, 2002.

BACKGROUND OF THE INVENTION

[0002] Spread spectrum OFDM (orthogonal frequency division multiplexing)multiple access, is one example of a spectrally efficient wirelesscommunications technology. OFDM can be used to provide wirelesscommunication services.

[0003] In OFDM spread spectrum system, the total spectral bandwidth isnormally divided into a number of orthogonal tones, e.g. subcarrierfrequencies. In a cellular network, the same bandwidth is often reusedin all the cells of the system. Those tones hop across the bandwidth forthe purpose of channel (frequency) diversity and interference averaging.Tone hopping follows predefined tone hopping sequences so that thehopped tones of a given cell do not collide with each other. The tonehopping sequences used in neighboring cells could be different toaverage interference between cells.

[0004] One exemplary form of the tone hopping sequences, is$\begin{matrix}{{F_{j}(t)} = {\frac{SLOPE}{\left\{ {{\frac{1}{j}{mod}\quad N} + t} \right\}}\quad {mod}\quad N}} & (1)\end{matrix}$

[0005] In the above equation, N is the total number of the tones, t isthe OFDM symbol index, j is the index of a tone hopping sequence, j=0, .. . , N−1, and F_(j)(t) is the index of the tone occupied by the j-thtone hopping sequence at time t. SLOPE is a cell specific parameter thatuniquely determines the tone hopping sequences used in a given cell.Neighboring cells could use different values of SLOPE.

[0006] Information (control and data) is transported via variousphysical channels. A physical channel corresponds to one or more tonehopping sequences defined in Equation (1). Therefore, those tone hoppingsequences are sometimes referred to as data tone hopping sequences. In aphysical channel, the basic transmission unit is a channel segment. Achannel segment includes the tones corresponding to the data tonehopping sequence(s) of the data channel over some time interval usuallycorresponding to a number of OFDM symbols.

[0007] In addition to the data tone hopping sequences, the OFDM spreadspectrum system may also use a pilot in a downlink to facilitate variousoperations, such operations may include synchronization and channelestimation. A pilot normally corresponds to one or more pilot tonehopping sequences. One exemplary form of a pilot tone hopping sequence,as disclosed in U.S. patent application Ser. No. 09/551,791, is

Pilot _(j)(t)=SLOPE·t+O _(j) mod N  (2)

[0008] By using different values for SLOPE, different pilot sequenceswill occur. Different pilot sequences may be used in different cells.

[0009] In the above equation, N, t, and SLOPE are the same parameters asused in Equation (1), j is the index of a pilot tone hopping sequence,Pilot_(j)(t) is the index of the tone occupied by the j-th pilot tonehopping sequence at time t, and O_(j) is a fixed offset number of thej-th pilot tone hopping sequence. Normally, the cells in a system usethe same set of offsets {O_(j)}.

[0010] In the OFDM spread spectrum system, the pilot and data tonehopping sequences are normally periodic with the same periodicity anduse the same value for parameter SLOPE. The time interval of one periodof a tone hopping sequence is sometimes referred to as a super slot.Thus, a super slot corresponds to a period after which a pilot sequencewill repeat. The structures of the pilot, physical channels, and channelsegments generally repeat from one super slot to another, and thereforecan be uniquely determined once the super slot boundaries have beenidentified.

[0011]FIG. 1 shows a frequency vs time graph 100 used to illustrategeneral concepts of data and pilot tone hopping sequences, control anddata traffic channels, channel segments, and super slots.

[0012]FIG. 1 includes a first row 102, a second row 104, a third row106, a fourth row 108, and a fifth row 110. Each row 102, 104, 106, 108,110 corresponds to a different orthogonal frequency tone in thefrequency domain.

[0013]FIG. 1 also includes a first column 112, a second column 114, athird column 116, a fourth column 118, a fifth column 120 a sixth column122, a seventh column 124, an eighth column 126, a ninth column 128, anda tenth column 130. Each column 112, 114, 116, 118, 120, 122, 124, 126,128, 130 corresponds to an OFDM symbol time in the time domain.

[0014] In the FIG. 1 example, super slots 133, 135 each have a periodequal to the period of the tone hopping sequence. First super slot 133has a period of five OFDM symbol times represented by first throughfifth columns 112, 114, 116, 118, 120 and defined by vertical timedomain boundary lines 111 and 121. Second super slot 135 also has aperiod of five OFDM symbol times. Super slot 135 corresponds to sixththrough tenth columns 122, 124, 126, 128, 130 and is defined by verticaltime domain boundary lines 121 and 131.

[0015] During the first super slot (columns 112, 114, 116, 118, 120),data tone hopping sequences are shown for a first traffic segment. Threetones are dedicated to the first traffic segment during each symbolperiod. The data tone hopping sequence for the first exemplary trafficchannel segment is illustrated by diagonal line shading which descendsin FIG. 1 from left to right. During the second super slot (columns 122,124, 126, 128, 130), data tone hopping sequences are shown for a secondtraffic segment. The data tone hopping sequence repeats in each superslot 133, 135. The data tone hopping sequence for the second exemplarytraffic channel segment is illustrated by ascending diagonal lineshading in FIG. 1. During the OFDM time intervals represented by firstcolumn 112 and the sixth column 122, the traffic channel data is shownto include frequency tones represented by first row 102, second row 104and third row 106. During the OFDM time intervals represented by secondcolumn 114 and the seventh column 124, the traffic channel data is shownto include frequency tones represented by first row 102, third row 106and fifth row 110. During the OFDM time intervals represented by thirdcolumn 116 and the eighth column 126, the traffic channel data is shownto include frequency tones represented by second row 104, fourth row 108and fifth row 110. During the OFDM time intervals represented by fourthcolumn 118 and the ninth column 128, the traffic channel data is shownto include frequency tones represented by first row 102, third row 106and fourth row 108. During the OFDM time intervals represented by fifthcolumn 120 and the tenth column 130, the traffic channel data is shownto include frequency tones represented by second row 104, third row 106and fourth row 108.

[0016]FIG. 1 also shows a pilot tone hopping sequence. The pilot tonehopping sequence repeats in each super slot 133, 135. The pilot tonehopping sequence is illustrated in FIG. 1 by use of small horizontalline shading. During the OFDM time intervals represented by first column112 and the sixth column 122, the pilot tone is assigned to thefrequency tone represented by fifth row 110. During the OFDM timeintervals represented by second column 114 and the seventh column 124,the pilot tone is assigned to the frequency tone represented by fourthrow 108. During the OFDM time intervals represented by third column 116and the eighth column 126, the pilot tone is assigned to the frequencytone represented by third row 106. During the OFDM time intervalsrepresented by fourth column 118 and the ninth column 128, the pilottone is assigned to the frequency tone represented by the second row104. During the OFDM time intervals represented by fifth column 120 andthe tenth column 130, the pilot tone is assigned to the frequency tonerepresented by the first row 102.

[0017] In some OFDM spread spectrum systems, the traffic channel isassigned in a segment-by-segment manner. Specifically, traffic channelsegments can be independently assigned to different wireless terminals.A scheduler determines the amount of transmission power and the burstdata rate, associated with a particular channel coding and modulationscheme, to be used in each traffic channel segment. The transmissionpowers and burst data rates of different traffic channel segments may bedifferent.

[0018] Sectorization is a popular method to improve wireless systemcapacity. For example, FIG. 2 illustrates a cell 200 including threesectors: sector 1 201, sector 2 203, and sector 3 205. Cell 200 alsoincludes a base station 207 employing a 3-sector antenna includingantenna sector 1 209, antenna sector 2 211, and antenna sector 3 213.The sectorized antenna provides some isolation between the sectors 201,203, 205. In an ideal system, the same spectrum can be reused in all thesectors 201, 203, 205 without interfering with each other, therebytripling the system capacity (over an omni cell) in the 3-sector systemshown in FIG. 2. Unfortunately, ideal signal separation is not possiblein the real world, which generally complicates the use of sectorizationin some systems.

[0019] In theory, integrating the sectorization into an OFDM spreadspectrum system should improve the overall system performance. Howeverinterference between the sectors due to the limited antenna isolationand reflection from objects can limit the actual capacity gains over anomni cell. Accordingly, it can be appreciated that there is a need formethods and apparatus which will allow sectorization to be used in OFDMsystems in a manner that will improve the capacity of such systemswithout many of the interference problems associated with sectorization.

SUMMARY OF THE INVENTION

[0020] In accordance with the invention, the same spectrum, e.g.,frequencies, may be reused in each of a cell's sectors in a sectorizedFDM system. In some embodiments of the invention, the sectors of a cellare synchronized in terms of tone frequencies, OFDM symbol timing, datatone hopping sequences, channel segments and super slot boundaries.Synchronization of fewer transmission characteristics or parameters isused in some embodiments. In fact, some features of the invention suchas beacon signals discussed below may be used with minimal or nofrequency synchronization between sectors of a cell.

[0021] In various embodiments symbol timing between sectors of a cell issubstantially synchronized, e.g., the symbol transmission start timesare synchronized to within the time duration of a cyclic prefix includedin transmitted symbols. As is know in the art, it is common to add acyclic prefix, e.g., a copy of a portion of the symbol so that the samedata is at both ends of the transmitted symbol. Cyclic prefixes providesome protection against timing errors and can be used as a buffer interms of amount of acceptable timing differences which may occur betweensectors.

[0022] Different cells in the system may, but need not, be synchronizedin regard to transmission characteristics such as frequency. In thesynchronized sector embodiment, for any control or data traffic channelin a given sector, there is a corresponding control or data trafficchannel in each of the other synchronized sectors of the same cell. Thecorresponding channels in the different sectors will have the sameconfiguration of frequency tones and time intervals, e.g., transmissionfrequencies and symbol transmission times. Channels are divided intosegments for transmission purposes. Thus, corresponding channels willhave corresponding channel segments. Because of the high level ofsynchronization between the sectors in the fully synchronized sectorembodiment, inter-sector interference is concentrated betweencorresponding channel segments in such an embodiment. Non-correspondingchannel segments see comparatively little inter-sector interferencebetween each other.

[0023] In some embodiments, the pilots used in each of the sectors of acell have the same value of SLOPE, but different offsets. This resultsin the repeating sequence of pilot tones being the same in each sector,but the starting point of the sequence being different in terms of time.Thus, at any point in time, the pilots in different sectors of a cellmay be different.

[0024] When the sectorized OFDM spread spectrum system is used in acellular network, in accordance with the invention, neighboring cellsmay use different values of SLOPE to determine the pilot and channeltone hopping sequences. The slope offset sets may be the same indifferent cells. Different cells need not, and are not necessarilysynchronized, in terms of tone frequencies, OFDM symbol timing, tonehopping sequences, channel segments or super slot boundaries.

[0025] In accordance with one feature of the invention, in someembodiments, the transmission power allocated to corresponding channelsegments of different sectors of a cell, if active, are substantiallythe same in each of the sectors. In such a case, the difference betweenthe transmission powers for the corresponding active channel segments inthe sectors of a cell are no more than Delta, where Delta is a valueused to control channel power differences between sectors. DifferentDeltas may be used for different channels. In one embodiment, for atleast one channel, Delta is set to be a constant, for example, zero. Inanother embodiment, Delta may be different from one group ofcorresponding channels to another, from one group of correspondingchannel segments to another, or as a function of burst data rates usedin corresponding channel segments or some other criteria. A schedulermay be used to coordinate the power allocation in the various sectors ofa cell in a centralized manner. In accordance with the invention, thedynamic range of the allocated power between the traffic channels in thesame sector may be large, while the dynamic range of the allocated poweracross corresponding traffic channels in the various sectors is limited.In some embodiments, the difference between corresponding channels ofdifferent sectors is kept to under less than 3 dB relative powerdifference for channel segments which are actively used in each of acells sectors.

[0026] In order to facilitate differentiation of the signalscorresponding to channel segments of different sectors, distinctscrambling bit sequences may, and sometimes are, used in differentsectors when generating transmit signals in the respective sectors. Thewireless terminal receiver may use a particular scrambling bit sequenceto selectively demodulate the signal from an intended sectortransmission of a base station. Alternatively, the wireless terminalreceiver may use multiple scrambling bit sequences to demodulate thesignals from multiple sector transmissions of a base station or frommultiple base stations simultaneously.

[0027] The channel condition of a wireless terminal may be described interms of being in one of two characteristic regions. In the firstregion, the SIR is not limited by inter-sector interference. When in thefirst region, the base station can increase the received SIR byallocating high transmission power and thereby provide an improved SIR.In the second region, the SIR is limited by the inter-sectorinterference, in which case, allocating high transmission power may notremarkably increase the received SIR since inter-sector interferencewill increase as channel power is uniformly increased in thecorresponding channel of each sector.

[0028] In some embodiments, the wireless terminal estimates its channelcondition characteristics and notifies the base station, such that thebase station can make sensible scheduling decisions in terms of powerand burst data rate allocation. The channel condition information mayinclude information distinguishing between inter-sector interference andother interference. In accordance with the invention, the base station'sscheduler may use the reported channel condition characteristics of thewireless terminals including power information, signal strength, and SIRto match wireless terminals to appropriate channels in each sector.Decisions on providing additional power or allocating segments for awireless terminal to a channel having high power can be made based onthe indication of inter-sector interference relative to otherinterference. In this manner, wireless terminals which can benefit fromhigher transmission power, e.g., those subject to low inter-sectorinterference, can be allocated to high power channels in a preferentialmanner over wireless terminals subject to comparatively highinter-sector interference. Assignment of high power channel segments canbe used to load balance the system, improve or optimize systemperformance and/or increase throughput capability by evaluating andreducing inter-sector and inter-cell interference.

[0029] In accordance with one embodiment of the invention, if a wirelessterminal is operating within a sector's cell boundary region andassigned a channel segment, the cell's scheduler may leave the tonescorresponding to the channel segment in the sector adjacent the boundaryregion unassigned to reduce or eliminate the inter-sector interference.In accordance with the invention, sectorization isolation betweenwireless terminals in non-sector boundary areas may be managed by thescheduler's selective assignment of channel segments corresponding tochannels with different power levels to different wireless terminals.Low power channels segments are normally assigned to wireless terminalsnear the transmitter while high power channels segments are assigned towireless terminals far from the base station. The number of low powerchannels in a sector normally exceeds the number of high power channelswith, in many cases, more of the sector's total transmission power beingallocated to the relatively few high power channels than the largenumber of low power channels.

[0030] The base station may frequently and/or periodically transmit abeacon signal, e.g., a relatively high power signal on one or a fewtones, over a period of time, e.g., one symbol period. Transmissionpower is concentrated on one or a small number of tones, e.g., the tonesof the beacon signal, during the beacon transmission. This highconcentration of power may involve allocating 80% or more of a sector'stotal transmission power in the beacon tones. In one embodiment, thebeacon signal is transmitted at a fixed OFDM symbol duration, forexample, the first or the last OFDM symbol, of a super slot and mayrepeat every super slot or every few super slots. In such a case, beaconsignals are used to indicate superslot boundaries. Therefore, once thetime position of the beacon signal has been located, the super slotboundaries can be determined. In accordance with the invention, beaconsignals may be assigned to perform different tasks, e.g., conveydifferent types of information. Beacons may be assigned to use fixedpredefined frequencies, the frequency itself may convey information,such as, e.g., boundaries of a frequency band or the frequency maycorrespond to an index number, such as e.g., sector index number. Otherbeacons may be assigned multiple or varying frequencies which may berelated to an index number or numbers used to convey information, suchas, a slope value used to determine the hopping sequence of the cellinto which the beacon is transmitted. The set of tones that carry highpower in the beacon signal may be selected from a predefined group ofbeacon tone sets depending on the information to be conveyed. Use ofdifferent beacon tone sets in the beacon signal can indicate certainsystem information, such as the values of SLOPE, boundaries of thefrequency band, and sector index.

[0031] In one embodiment of the invention, the type of beacontransmitted varies as a function of transmission time, e.g., alternatesin the time domain. In another embodiment of the invention, the beaconfrequency tone assignments may be reconfigured if a failure or problemoccurs at a specific tone frequency. By utilizing both the time andfrequency domain to vary the beacon signal transmissions and theinformation conveyed, a large amount of information may be conveyed tothe mobiles in an efficient manner. This information may be used, e.g.,to determine the sector/cell location of the mobile, offload some of thefunctions required by the pilot such as e.g. synchronization tosuperslot boundaries, reduce the time required for pilot punch through,evaluate reception strength, and provide useful information to predictand improve the efficiency of hand-offs between sectors and cells.

[0032] In accordance with the invention, in some embodiments, thefrequency, symbol timing, and super slot structures of an uplink signalare slaved to those of the downlink signal, and are synchronized in thevarious sectors of a cell. In one embodiment, the data tone hoppingsequences and channel segments are synchronized across each of thesectors of a cell. In another embodiment, the data tone hoppingsequences and channel segments are randomized across each of the sectorsof a cell such that a channel segment in one sector may interfere withmultiple channel segments in another sector of the same cell.

[0033] One embodiment of the beacon features of the invention isdirected to a method of operating a base station transmitter in afrequency division multiplexed communications system. The base stationtransmitter uses a set of N tones to communicate information over afirst period of time using first signals, said first period of timebeing at least two seconds long, where N is larger than 10, and wherethe method includes transmitting during a second period of time a secondsignal including a set of X tones, where X is less than 5, and where atleast 80% of a maximum average total base station transmission powerused by said base station transmitter during any 1 second period duringsaid first period of time is allocated to said set of X tones. The firstperiod of time may be a large time interval, e.g., several minutes,hours or days. In some cases the first period of time is at least 30minutes long. In particular implementations X is equal to one or two.The second period of time may be a period of time, e.g., a symboltransmission period in which a beacon signal is transmitted. In somecases during the second period of time at least half of the N-X toneswhich are in said set of N tones but not in said set of X tones gounused during said second period of time. In some implementations noneof the N-X tones in said set of N tones but not in said set of X tonesare used during said second period of time. In other implementationsmultiple ones of the N-X tones in said set of N tones but not in saidset of X tones are used during said second period of time. The basestation may be part of a communications system which is an orthogonalfrequency division multiplexed system. In some OFDM implementations thesecond period of time is a period of time used to transmit an orthogonalfrequency division multiplexed symbol. The second period of time, e.g.,the beacon transmission period, may periodically repeat during saidfirst period of time. The method in this example may also includetransmitting during a third period of time a third signal including aset of Y tones, where Y<N, each tone in said third set of Y tones having20% or less of said maximum average total base station transmissionpower used by said base station transmitter during any 1 second periodduring said first period of time, said third period of time having thesame duration as said second period of time. The third period of timemay be, and in some embodiments is, a symbol time in which data signals,pilot signals and/or control signals are transmitted. The third periodof time may be different from the second period of time or overlap thesecond period of time. When the third period of time overlaps or is thesame as the second period of time, a small portion of the total powertransmitted during the period of time is available for use by the data,pilot and/or control signals which are modulated on the Y tones, e.g.,20% or less due to the consumption of at least 80% power by the beaconsignal(s), e.g., high power tone or tones. The high power tones, e.g.,one or more beacon tones, may be and in various embodiments are,transmitted at a predetermined fixed frequency. The predeterminedfrequency may, and often does, have a fixed frequency offset>0 from thelowest frequency tone in said set of N tones. This allows the beaconsignal to provide an indication of the boundary of the set of N tones.

[0034] In various embodiments at least one of said X tones, e.g., beacontones, is transmitted at a frequency which is determined as a functionof at least one of a base station identifier and a sector identifier. Inmany implementations, for each repetition of said second period of timein said first period of time there are at least Z repetitions of saidthird period of time in said first period of time where Z is at least10, e.g., there are many more data transmission symbol time periods thanbeacon signal symbol time periods. In some cases Z is at least 400,e.g., there are at least 400 data transmission symbol times for eachbeacon transmission signal time. In some implementations during a fourthperiod of time a fourth signal including G tones is transmitted, where Gis less than 5, and where at least 80% of said maximum average totalbase station transmitter power used by said base station transmitterduring any 1 second period during said first period of time is allocatedto said G tones. The G tones may correspond, e.g., to a symboltransmission time in which a different beacon signal from the onetransmitted in the second period of time is transmitted. In oneembodiment the frequency of at least one of said G tones is a functionof at least one of a base station identifier and a sector identifier,and said at least one of said G tones is not one of said set of X tones.In various implementations the second and fourth periods of timeperiodically repeat during said first period of time. In someembodiments, a base station includes a transmitter control routine whichincludes modules, e.g., software modules or blocks of code, whichcontrol the generation and transmission of the signals during each ofthe first, second, third and fourth transmission periods. A separatecontrol module may not be used for the first signal period when it isfully comprised of second, third and fourth signal transmission periodswith the control modules for these periods control transmission.Accordingly, transmission control means may include one or more softwaremodules with each software module controlling a different transmissionfeature, e.g., a separate transmission feature of the invention recitedin one of the pending claims. Thus, while a single transmitter controlroutine may be present in a base station, the single routine may, andoften does, include multiple different control modules.

[0035] A communication method for use in a base station of a sectorizedcell which is directed to various synchronization features of theinvention will now be described. In accordance with the method the basestation transmits symbols, e.g., modulated symbols, into multiplesectors of said cell using orthogonal frequency division multiplexedsymbols. The frequency division multiplexed symbols are generated bymodulating information on one or more symbols and, in most cases, addinga cyclic prefix to the form the modulated symbol to be transmitted. Themethod comprises, in one embodiment, operating each sector to use a setof tones to transmit orthogonal frequency division multiplexed symbols,each orthogonal frequency division multiplexed symbol. The symbols aretransmitted at symbol transmission start times. Thus, each transmittedsymbol has a symbol transmission start time. In accordance with theinvention each sector is controlled to use the same set of tones, thesame duration of each symbol transmission period, and substantially thesame symbol start times. In various embodiments each of said orthogonalfrequency division multiplexed symbols include a cyclic prefix having acyclic prefix length. In some of these embodiments substantially thesame symbol transmission start times are such that the differencebetween the symbol transmission start times of any two adjacent sectorsare at most the amount of time used to transmit a cyclic prefix. A setof hopping sequences is often used to allocate tones to a first set ofcommunication channels in a first sector of said cell. The same set ofhopping sequences is used to allocate tones to a corresponding set ofcommunication channels in each of the other sectors of the cell. Eachhopping sequence has a start time. The start time of each hoppingsequence in said set of hopping sequences is the same in each of saidsectors in one embodiment. In order to allow devices to distinguishbetween signals corresponding to different sectors of a cell withdifferent information to be transmitted, e.g., modulated symbols, may besubject to a scrambling operation prior to transmission. Differentscrambling sequences are used in different sectors. Thus, the scramblingsequence provides a way of distinguishing between data corresponding todifferent sectors. Thus, in at least one embodiment, scrambling ofmodulation symbols is performed prior to transmitting said modulationsymbols using said transmitted symbols with a different scramblingsequence being used in each sector of the cell. The communicationchannels in each of the sectors of a cell are normally partitioned intosegments, segments of corresponding channels in each of the sectors ofthe cell have the same segment partitions and have segment start timeswhich are substantially the same, such that for a segment of a channelin one sector there is another segment of the corresponding channelwhere the two segments use the same set of hopping sequences and thesame segment start times. In some embodiments the segment start timesfor segments of the same channel in different cells differ by no morethan the time used to transmit a cyclic prefix. Pilot tones are oftentransmitted in each sector of the cell. In various embodiments themethod of the invention includes transmitting a portion of pilot tonesin each sector of the cell according to a pilot tone hopping sequence,the same pilot tone hopping sequence being used in each sector but witha different fixed tone offset being used in each of the sectors of acell. The pilot tone hopping sequence may be a slope hopping sequence.In such implementations, adjacent cells can use different slope valuesfor determining the slope hopping sequences to be used. In someimplementations, pilot tones in each sector of the cell are transmittedaccording to a set of pilot tone hopping sequences, the same set ofpilot tone hopping sequences being used in each sector but withdifferent fixed tone offsets being used in each of the sectors of thecell. In such a case, pilot tone hopping sequences in a set of pilottone hopping sequences corresponding to a sector are often offset fromeach other by a corresponding preselected set of offsets, thecorresponding preselected set of offsets being the same in each sectorof the cell. Furthermore in such a case the set of pilot tone hoppingsequences used in any two adjacent sectors of the cell may not beidentical due to the use of different fixed tone offsets in the adjacentsectors. The set of pilot tone hopping sequences being used in any twoadjacent sectors of the cell need not be, and sometimes are notidentical, due to the use of different fixed tone offsets in theadjacent sectors for the pilot tone hopping sequences.

[0036] The power control methods of the present invention can be usedalone or in combination with the other features and/or methods of theinvention. In accordance with an exemplary power control method of theinvention, a set of tones is used in a cell. A transmitter in the celltransmits into a first sector of said cell over a plurality of symboltimes using tones from said set of tones. The cell includes a secondsector adjoining said first sector. The transmitter transmits into saidsecond sector on first and second communications channels, the firstcommunications channel including a first subset of said set of tonesduring each of a first subset of said plurality of symbol times, thesecond communications channel including a second subset of said set oftones during each of said first subset of said plurality of times, saidfirst subset of said set of tones and said second subset of said set oftones being different from each other during each symbol time. In onesuch implementation, the exemplary method includes operating thetransmitter to transmit on said first and second channels into saidfirst sector in a synchronous manner with transmissions made by saidtransmitter into said second sector; and controlling a totaltransmission power of the tones corresponding to the first channel inthe first sector during said first subset of said plurality of symboltimes to be greater than 20% and less than 500% of a total power of thetones corresponding to the first channel transmitted into the secondsector, during said first subset of said plurality of symbol times. Insome implementations controlling the total transmission power of thetones corresponding to the first channel includes limiting the totalpower used in said first subset of symbol times to be no more than afixed fraction of a maximum average total transmission power used bysaid transmitter in the first sector during any 1 hour period, saidfixed fraction also being used to limit the total transmission power ofthe tones corresponding to the first channel in the second sector duringthe first subset of symbol times to be no more than said fixed fractionof a maximum average total transmission power used by said transmitterin the second sector during any 1 hour period, said fixed fraction beingless than 100%. The symbol times are, in some implementations,orthogonal frequency division multiplexed symbol transmission timeperiods. In such cases the tones are normally orthogonal frequencydivision tones. The set of tones may be, and often is, different duringat least two symbol times. Symbols transmitted at different times maycorrespond to different symbol constellations. In some implementations,said transmitter transmits into said first sector symbols correspondingto a first constellation on said first channel during said first subsetof symbol times and transmits symbols corresponding to a secondconstellation during a second subset of said plurality of symbol times,the second constellation including more symbols than the firstconstellation, in such a case, the method includes controlling a totaltransmission power of the tones corresponding to the first channel inthe first sector during the second subset of said plurality of symboltimes to be greater than 50% and less than 200% of a total power of thetones transmitted in the second sector corresponding to the firstchannel during said second subset of said plurality of symbol times. Inanother embodiment the transmitter transmits into the first sectorsymbols at a first channel coding rate on said first channel during saidfirst subset of said plurality of symbol times and transmits symbols ata second channel coding rate during a second subset of said plurality ofsymbol times, said second channel coding rate being higher than saidfirst channel coding rate. In such an implementation, the method furthercomprises controlling a total transmission power of the tonescorresponding to the first channel in the first sector during the secondsubset of said plurality of symbol times to be greater than 50% and lessthan 200% of a total power of the tones transmitted in the second sectorcorresponding to the first channel during said second subset of saidplurality of symbol times. The total transmission power of thetransmitted tones corresponding to the first channel in the first sectorduring the first subset of said plurality of symbol times may be, and insome implementations is, equal to the total transmission power of thetransmitted tones in the first channel in the second sector during saidfirst subset of said plurality of symbol times. In many cases, the firstsubset of said plurality of symbol times will include many, e.g., atleast 14, consecutive symbol times. The method further comprisescontrolling the total power of the tones transmitted in the first sectorcorresponding to the first channel during a fourth subset of saidplurality of symbol times to be one of greater than 200% and less than50% of the total power of the tones transmitted in said first sectorcorresponding to the second channel during said fourth subset of saidplurality of symbol times. In some implementations the power controlmethod includes controlling the total power of the tones transmitted inthe first sector corresponding to the first channel during a fourthsubset of said plurality of symbol times to be one of greater than 200%and less than 50% of the total power of the tones transmitted in saidfirst sector corresponding to the second channel during said fourthsubset of said plurality of symbol times. The fourth subset of saidplurality of symbol times sometimes includes at least 14 consecutivesymbol times and in some cases more than 40. In some implementations thefirst and second sectors use a third communications channel during asecond subset of said plurality of symbol times, the thirdcommunications channel includes a third subset of said set of tonesduring each of said second subset of said plurality of symbol times. Insuch a case the power control method often further includes the step ofcontrolling the transmitter during said second subset of said pluralityof symbol times, to limit the total transmission power on tonescorresponding to said third communications channel transmitted by saidtransmitter to be less than 10% of the total transmission power used bysaid transmitter to transmit tones into said second sector correspondingto the third channel during said second subset of said plurality ofsymbol times. In some cases, to limit interference e.g., between sectorsfor segments used to transmit control signals, the method includescontrolling the transmitter during said second subset of said pluralityof symbol times, to limit total transmission power on tonescorresponding to said third communications channel transmitted by saidtransmitter to be zero. In various implementations, the method of theinvention is further directed to controlling the allocation ofresources, e.g., segments, corresponding to the third communicationschannel to wireless terminals. In such an implementation the methodincludes operating the base station or an apparatus included therein toidentify wireless terminals in a boundary area which corresponds to aboundary between said first and second sectors; and to allocate theresources, e.g., channel segments, corresponding to the said thirdchannel to at least one of said identified wireless terminals.Identifying wireless terminals in the boundary region may includereceiving from a wireless terminal first information indicating anamount of intersector interference measured by said wireless terminaland second information indicating an amount of background interferencemeasured by said wireless terminal. Identifying wireless terminals inboundary regions may alternatively or in addition, include receiving asignal, e.g., a location signal, from a wireless terminal in saidboundary area a signal indicating that said wireless terminal is in saidboundary area. In some power control embodiments, the first and secondsectors use said third communications channel during a third subset ofsaid plurality of symbol times, said third subset of said plurality ofsymbol times being different from said second subset of said pluralityof symbol times. In such a case, the method may further comprisecontrolling said transmitter during said third subset of said pluralityof symbol times, to use a total transmission power on tonescorresponding to said third communications channel transmitted by saidtransmitter into the first sector to be at least 1000% used by saidsecond sector to transmit tones corresponding to the third channel intothe second sector during said third subset of said plurality of symboltimes. This 1000% represents power 10 times that used in the secondsector. This power difference will often be sufficient to makeintersector interference seen in the first sector to be a relativelysmall component of signal interference. In some implementations saidfirst and second sector use said third communications channel during athird subset of said plurality of symbol times, said third subset ofsaid plurality of symbol times being different from said second subsetof said plurality of symbol times. In one such implementation the methodfurther includes: controlling said transmitter during said third subsetof said plurality of symbol times, to use a total transmission power ontones corresponding to said third communications channel transmitted bysaid transmitter into the first sector to be at least 1000% used by saidsecond sector to transmit tones corresponding to the third channel intothe second sector during said third subset of said plurality of symboltimes. In the power control implementations just discussed, a basestation control routine may include different segments of code toperform each of the recited control operations. Furthermore, whileantennas or other elements of the base station transmitter may bedifferent in each sector, in many implementations the common controllogic and control functionality associated with the base station isresponsible for controlling transmission in various sectors inaccordance with one or more features of the invention.

[0037] Additional features, benefits and embodiments of the presentinvention are discussed in the detailed description which follows.

BRIEF DESCRIPTION OF THE FIGURES

[0038]FIG. 1 illustrates the general concepts of data and pilot tonehopping sequences, control and data traffic channels, channel segments,and super slots.

[0039]FIG. 2 shows a three sector cell with a base station employing a 3sector antenna.

[0040]FIG. 3 shows a three sector cell with a base station illustratingthe concept of inter-sector boundary interference regions.

[0041]FIG. 4 illustrates an exemplary communications system utilizingcell sectorization in accordance with the present invention.

[0042]FIG. 5 illustrates an exemplary access node that may be used inthe communication system of FIG. 4 in accordance with the presentinvention.

[0043]FIG. 6 illustrates an exemplary end node that may be used in thecommunications system of FIG. 4 in accordance with the presentinvention.

[0044]FIG. 7 illustrates frequency tone synchronization throughout thesectors of a cell in accordance with the present invention.

[0045]FIG. 8 illustrates OFDM symbol time synchronization throughout thesectors of a cell in accordance with the present invention.

[0046]FIG. 9 illustrates that in all the sectors of a cell, the tonefrequencies occupied by the j-th tone hopping sequence at any OFDM timeare identical and that the super slot boundaries are identical inaccordance with the present invention. FIG. 9 further illustrates theconcept of corresponding control or data channel segments within thesectors of a cell in accordance with the present invention.

[0047]FIG. 10 shows an exemplary case where the frequency tones aredistributed amongst two traffic channels. For each control or datatraffic channel, the tone hopping sequence at any OFDM time is identicalacross the three exemplary sectors of the cell in accordance with thepresent invention.

[0048]FIG. 11 illustrates exemplary pilot tone hopping sequences withthe same slope value but a different offset value in each sector of acell in accordance with the present invention.

[0049]FIG. 12 illustrates the concept of the pilot tone hopping sequenceof FIG. 111 puncturing the data sequence of FIG. 10 in accordance withthe present invention.

[0050]FIG. 13 shows a table illustrating exemplary power allocationbetween different traffic channel segments in the same sector of a celland across the corresponding traffic channel segments in all the sectorsof a cell in accordance with one embodiment of the present invention.

[0051]FIG. 14 shows a graph of per tone power vs frequency tone forordinary OFDM signal.

[0052]FIG. 15 shows a graph of per tone power vs frequency tone for thetime of beacon signal transmission where the total power is concentratedon just two tones in accordance with one implementation of the presentinvention.

[0053]FIG. 16 shows a graph of per tone power vs frequency tone for thetime of beacon signal transmission where the total power is concentratedon just one tone in accordance with one implementation of the presentinvention.

[0054]FIG. 17 shows a graph of per tone power vs frequency tone for thetime of beacon signal transmission illustrating a predefined group ofbeacon tone sets in accordance with one embodiment of the presentinvention.

[0055]FIG. 18 shows a graph of frequency vs OFDM symbol timeillustrating the concept of different functionality for successivebeacons in the time domain in accordance with one embodiment of thepresent invention.

[0056]FIG. 19 shows a graph of frequency vs OFDM symbol timeillustrating the concept of transmitting alternating beacons types inthe time domain in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0057] With the OFDM spread spectrum system, the tones used in a givencell are all orthogonal. Therefore, the data hopping sequences and thephysical channels do not interfere with each other. Given the wirelesschannel propagation characteristics, depending on its location, awireless terminal may experience a large dynamic range of channelconditions measured in terms of signal-to-interference ratio (SIR) orsignal-to-noise ratio (SNR). Such a property can be exploited to enhancethe system capacity. For example, in accordance with the invention, ascheduler may optimally balance the power allocation in the trafficchannel by serving simultaneously wireless terminals with dramaticallydifferent wireless channel conditions. In that case, a wireless terminalwith a bad wireless channel condition may be allocated with a largeportion of transmission power and possibly a small portion of bandwidththereby gaining service robustness, while another wireless terminal witha good wireless channel condition may be allocated with a small portionof transmission power and possibly a large portion of bandwidth and canstill achieve a high burst data rate.

[0058] The OFDM spread spectrum system of the invention can be combinedwith the sectorized antenna to improve the overall system performance.However, in reality, antenna isolation is never perfect. A signaltransmitted in one sector may leak to another sector with an attenuationfactor, thereby causing interference between sectors, i.e., inter-sectorinterference. The inter-sector interference may reduce the gains ofpower and burst data rate allocation. For example, in the absence of theinter-sector interference, a wireless terminal with a good wirelesschannel condition may be allocated with a small portion of transmissionpower and can still achieve high burst data rate. In the presence of theinter-sector interference, the wireless terminal may not achieve thesame high burst data rate with the same amount of transmission power.The situation becomes especially severe when the inter-sectorinterference comes from a traffic channel that is transmitted at muchhigher power, for example to serve another wireless terminal with badchannel condition.

[0059]FIG. 3 illustrates an exemplary cell 300 including 3 sectors:sector 1 301, sector 2 303, and sector 3 305 and a base station 307including a 3 sector antenna. The base station 307 may communicate withend nodes, e.g. mobile nodes or mobile terminals, situated at arbitrarylocations within the cell 300 via wireless links. From an interferenceperspective, cells may be deemed to be comprised of sector boundaryareas where interference from a neighboring sector may be a severeproblem and non-sector boundary areas. In the FIG. 3 illustration of thecell 300, the non-sector boundary areas are distinguished from theboundary areas. The cell 300 includes non-sector boundary area 1 309,non-sector boundary area 2 311, and non-sector boundary area 3 313. Thecell 300 also includes sector boundary areas: sector 1-2 boundary area315, sector 2-3 boundary area 317, and sector 3-1 boundary area 319. Thelevel of sectorization isolation can be described in terms of the amountof leakage between the non-sector boundary areas 309, 311, and 313. Forexample if a mobile node is situated in non-sector boundary area 1 309leakage may occur from signal intended for sector 2 303 and signalintended for sector 3 305. The leakage in the non-sector boundary areas309, 311, 313, is typically −13 dB to −15 dB, and will depend on factorssuch as the base station 307 antenna type. In the sector boundaryregions (sometimes referred to as 0 dB regions), areas 315, 317, and 319the signal strength at the reception point, may be almost equivalentfrom the two adjacent sector antennas. The present invention describesmethod and apparatus to improve the capacity of the system when deployedin a sectorized configuration.

[0060] For the purpose of illustration and description, a 3-sector cell300 is used in FIG. 3 and in the subsequent examples of FIGS. 7, 8, 9,10, 11, 12, and 13. However, it is to be understood that the presentinvention is applicable to other sectorization scenarios. In asectorized cell, the sectors are indexed. For example, in the 3-sectorcell 300 of FIG. 3, the sector indices can be 1, 2, and 3.

[0061]FIG. 4 illustrates an exemplary communications system 400employing cell sectorization and wireless communication in accordancewith the present invention. The communications system 400 includes aplurality of cells, cell 1 438, cell N 440. Cell 1 438 represents thecoverage area for access node (AN) 1 402 located within cell 1 438. Theaccess node 1 402 may be, for example, a base station. Cell 1 438 issubdivided into a plurality of sectors, sector 1 442, sector Y 444. Adashed line 446 represents the boundaries between sectors 442, 444. Eachsector 442, 444 represents the intended coverage area corresponding toone sector of the sectorized antenna located at the access node 1 402.Sector 1 442 includes a plurality of end nodes (ENs), EN(1) 422, EN(X)424 coupled to AN 1 402 via wireless links 423, 425, respectivley.Similarly, sector Y 444 includes a plurality of end nodes, EN(1′) 426,EN(X′) 428 coupled to AN 1 402 via wireless links 427, 429,respectively. The ENs 422, 424, 426, 428 may be, e.g., mobile nodes ormobile terminals and may move throughout the system 400.

[0062] Cell N 440 is subdivided into a plurality of sectors, sector 1448, sector Y 450 with sector boundaries 446′. Cell N 440 is similar tocell 1 438 and includes an access node M 402′, and a plurality of ENs422′, 424′, 426′, 428′ coupled to AN M 402′ via wireless links 423′,425′, 427′, 429′, respectively.

[0063] The access nodes 402, 402′ are coupled to a network node 406 vianetwork links 412, 414, respectively. The network node 406 is coupled toother networks nodes, e.g. other access nodes, intermediate node, HomeAgent Nodes or Authentication, Authorization Accounting (AAA) servernodes, via network link 420. The network links 412, 414, 420, may be,for example, fiber optic cables.

[0064]FIG. 5 illustrates an exemplary access node 500 of the presentinvention that may be used in the communications system 400 of FIG. 4,e.g., AN1 402 of FIG. 4. The access node 400 includes a processor 502,e.g., CPU, a wireless communications interface 504, anetwork/Internetwork interface 506, and a memory 508. The processor 502,wireless communications interface 504, network/Internetwork interface506, and memory 508 are coupled together by a bus 510 over which theelements 502, 504, 506, 508, can exchange data and information.

[0065] The processor 502 controls the operation of the access node 500by executing routines and utilizing data within the memory 528 in orderto operate the interfaces 504, 506, perform the necessary processing tocontrol basic functionality of the access node 500 and to implement thefeatures and improvements employed in the sectorized system inaccordance with the present invention.

[0066] The wireless communications interface 504 includes a receivercircuit 512 and a transmitter circuit 514 coupled to sectorized antennas516, 518, respectively. The receiver circuit 512 includes a Descramblercircuit 520 and the transmitter circuit 514 includes a scrambler circuit522. The sectorized antenna 516 receives signals from one or more mobilenodes, e.g. EN1 422 of FIG. 4. The receiver circuit 512 processes thereceived signals. The receiver circuit 512 uses its descrambler 520 toremove the scrambling sequence if scrambling was used duringtransmission by the mobile node. The transmitter circuitry 514 includesa scrambler 522 which may be used to randomize the transmitted signal inaccordance with the present invention. The access node 500 may transmitsignal to the mobile nodes, e.g. EN1 422 of FIG. 4, over its sectorizedantenna 518.

[0067] The network/internetwork interface 506 includes a receivercircuit 524 and a transmitter circuit 526 which will allow the accessnode 500 to be coupled to other network nodes, e.g. other access nodes,AAA servers, Home Agent Nodes, etc. and interchange data and informationwith those nodes via network links.

[0068] The memory includes routines 528 and data/information 530. Theroutines include signal generation routines 532 and a scheduler 534. Thescheduler 534 includes various routines such as an inter-sectorinterference routine 536, an inter-cell interference routine 538, apower allocation routine 540, and a wireless terminal/traffic segmentmatching routine 542. The data/information 530 includes data/controlinformation 544, pilot information 546, beacon information 548, tonefrequency information 550, OFDM signal timing information 552, data tonehopping sequences 554, channel segments 556, super slot boundaryinformation 558, slope values 560, pilot values 562, delta 564, burstdata rates 566, MN channel condition information 568, power information570, and MN sector information 572. The tone frequency information 550includes sets of tones used for different signals: set of N tones usedfor OFDM signals, sets of X tones used for some beacon signals, sets ofY tones used for OFDM signals, and sets of G tones used for other beaconsignals, and repetition rate information associated with the varioussets of tones. Power information 570 includes wide and narrowinter-sector transmission power control range information, inter-channeltransmission power allocation range information, boundary transmissionpower range information, and power levels allocated for the channels ineach sector.

[0069] The signal generation routines 532 utilize the data/info 530,e.g., super slot boundary information 558, tone frequency information550, and/or OFDM symbol timing information 552, to perform signalsynchronization and generation operations. Signal generation routine 532also utilizes the data/info, e.g., the data tone hopping sequences 554,data/control info 544, pilot info 546, pilot values 562, and/or sectorinformation 572 to implement data/control hopping and pilot hoppingsequences. In addition signal generation routine 532 may utilizedata/info 530, e.g., beacon info 530, to generate beacon signals inaccordance with the present invention.

[0070] The inter-sector interference routine performs operations usingthe methods of the present invention and the data/info 530, such as,pilot info 546, MN channel condition information 568, and MN sectorinformation 572 to evaluate and reduce inter-sector interference withina given cell. The inter-cell interference routine 536 utilizes themethods of the present invention and data/info 530, e.g., reported MNchannel condition information 568, and slope values 560, to evaluate andreduce the effects of inter-cell interference. The power allocationroutine 540 uses the methods of the present invention and data info,e.g. power info 570 and delta 564, to control the power allocation tothe various traffic channels, e.g., to optimize performance. Thewireless terminal/traffic and segment matching routine 542 uses thedata/info 530, e.g. MN channel condition information 568, powerinformation 570, channel segments 556, and burst data rates 566 toassign wireless terminals as a function of their power needs to be in anappropriate channel segment in accordance with the invention.

[0071] Various specific functions and operations of the access node 500will be discussed in more detail below.

[0072]FIG. 6 illustrates an exemplary end node (EN) 600, e.g. a wirelessterminal such as mobile node (MN), mobile, mobile terminal, mobiledevice, fixed wireless device, etc., that may be used in the exemplarycommunications system 400 of FIG. 4 in accordance with the presentinvention. In this application, at various locations, references may bemade to the end node using various terminology and various exemplaryembodiments of the end node such as, e.g., wireless terminal, mobilenode, mobile, mobile terminal, fixed wireless device, etc.; it is to beunderstood that the apparatus and methods of the invention are alsoapplication to the other embodiments, variations and descriptions of theend node. The end node 600 includes a processor 602, e.g., CPU, awireless communications interface 604, and a memory 606. The processor602, wireless communications interface 604, and memory 606 are coupledtogether by a bus 608 over which the elements 602, 604, and 606, caninterchange data and information.

[0073] The processor 602 controls the operation of the end node 600 byexecuting routines and utilizing data within the memory 606 in order tooperate the wireless communications interface 604, perform the necessaryprocessing to control basic functionality of the end node 600 whileimplementing the features and improvements employed in the sectorizedsystem in accordance with the present invention.

[0074] The wireless communications interface 604 includes a receivercircuit 610 and a transmitter circuit 612 coupled to antennas 614, 616,respectively. The receiver circuit 610 includes a Descrambler circuit618 and the transmitter circuit 612 includes a scrambler circuit 620.The antenna 614 receives broadcast signals, e.g., from an access node,e.g. AN1 402 of FIG. 4. The receiver circuit 610 processes the receivedsignal and may use its descrambler 618, e.g., decoder, to removescrambling if scrambling was used during transmission by the accessnode. The transmitter circuitry 612 includes a scrambler 620, e.g.,encoder, which may be used to randomize the transmitted signal inaccordance with the present invention. The end node 600 may transmit theencoded signal to the access node over its antenna 616.

[0075] The memory 606 includes routines 622 and data/information 624.The routines 622 include hopping sequence routines 626, a channelcondition monitoring/reporting routine 628, and a beacon signal routine630. The data/information 624 includes MN channel condition information632, power information 634, tone frequency information 636, OFDM signaltiming information 638, data tone hopping sequences 640, channelassignment information 642, super slot boundary information 644, slopevalues 646, pilot values 648, slope indexes 650, beacon info 652, sectoridentification 654, and cell identification 656.

[0076] The hopping routines 626 include a data/control hopping sequenceroutine 634 and a pilot hopping sequence routine 632 which performsoperations using the methods of the present invention and the data/info624, such tone frequency info 636, OFDM signal timing information 638,data tone hopping sequences 640, channel assignment information 642,super slot boundary information 644, slope values 646, and/or pilotvalues 648 to process the received data, identify the cell 656 andsector 654 that the mobile 600 is operating in and the correspondingaccess node 500 of FIG. 5 that is communicating with the end node 600.The channel condition monitoring/reporting routine 628 performsoperations using the methods of the present invention and data info 624,e.g., MN channel condition info 632, power info 634, and channelassignment 642 to evaluate the status and quality of the wireless linkto the access node 500 and subsequently report that data back to theaccess node 500 for use in scheduling. The beacon signal routine 630performs operations relating to beacon signals in accordance with themethods of the present invention. Beacon signal routine 630 uses thedata/info 624, e.g. beacon info 652, power info 634, tone frequency info636, super slot boundaries 644, and/or slope indexes 650 to performfunctions such as, e.g., synchronization of super slot boundaries,determine boundaries of frequency band and sector index 654, determineslope value 646, determine cell location 656 and pilot values 648.

[0077] Various specific functions and operations of the end node 600will be discussed in more detail below.

[0078] Physical layer full synchronization across the sectors will nowbe described.

[0079] In accordance with the invention, the same spectrum is reused ineach of the sectors in a cell of the sectorized OFDM spread spectrumsystem. Moreover, in accordance with one particular exemplary embodimentof the invention, each of the sectors of a cell are fully synchronizedin terms of tone frequencies, OFDM symbol timing, data tone hoppingsequences, channel segments and super slot boundaries. While suchsynchronization is desirable, aspects of the invention may be used insystems where synchronization between sectors in a cell is not socomplete as in the case of the particular exemplary embodiment.Specifically, in each of the sectors of a cell the same set of tones isused with identical sets of tone frequencies being included in each set.The OFDM symbol timings are also identical. FIG. 7 700 illustrates thesets of the tone frequencies used in each of 3 sectors which form acell. The horizontal axis 707 of FIG. 7 corresponds to frequency. Eachvertical arrow represents a frequency tone.

[0080] Rows 701, 703, 705 each correspond to a different sector of theexemplary cell. The same set of N tones is used in each sector, with thetones used in each sector being indexed 0 through N−1.

[0081]FIG. 8800 illustrates OFDM symbol timing used in the 3 sectors.The horizontal axis 807 of FIG. 8 represents how time can be divided ineach sector according to symbol times, e.g., the time used to transmitan OFDM symbol. Each division on the horizontal axis 807 marks the startof a new symbol time in each of the sectors of a cell. Row 1 (801)corresponds to symbol times in sector 1 while rows 2 and 3 (803,805)correspond to symbol times in sectors 2 and 3 of the same cell. Notethat symbol start times are synchronized in the three sectors of thecell. Each of the sectors of the cell derive the data tone hoppingsequences using the same OFDM symbol index and the same value of SLOPEin Equation (1). Therefore, in each of the sectors, the tone frequenciesoccupied by the j-th tone hopping sequence at any OFDM time areidentical and the super slot boundaries are also identical.

[0082] Furthermore, the physical layer channels and channel segments areconstructed in the same way in each of the sectors in the exemplarycell. FIG. 9 shows a frequency vs time graph 900 to illustrate thecontrol and data traffic channels and channel segments in the 3 sectorsof the exemplary cell shown in FIG. 3.

[0083]FIG. 9 illustrates the transmission of symbols in each of the 3sectors of the exemplary cell shown in FIG. 3 during a single superslot.In the FIG. 9 example, each horizontal division corresponds to a symboltransmission time where the exemplary superslot corresponds to 5 symboltimes.

[0084] In the FIG. 9 example, a super slot 943, the time interval of oneperiod of the data/control tone hopping sequence, is shown as theconcatenation of five OFDM symbol times, represented by first throughfifth columns 932, 934, 936, 938, 940 and defined by vertical timedomain boundary lines 931 and 941.

[0085]FIG. 9 includes a first group of first through fifth rows 902,904, 906, 908, and 910 which correspond to a first sector of the cell.Each row 902, 904, 906, 908, 910 corresponds to a different orthogonalfrequency tone in the frequency domain of sector 1.

[0086] A second group of first through fifth rows 912, 914, 916, 918,and 920 corresponds to a second sector of the cell. Each row 912, 914,916, 918, 920 corresponds to a different orthogonal frequency tone inthe frequency domain of sector 2.

[0087] A third group of first through fifth rows 922, 924, 926, 928, and930 corresponds to a third sector of the cell. Each row 922, 924, 926,928, 930 corresponds to a different orthogonal frequency tone in thefrequency domain of sector 3.

[0088] The same frequency tone is represented by first row 902 forsector 1, the first row 912 for sector 2, and the first row 922 forsector 3. Similarly, frequency tone equivalency exists across the threesectors for the following sets: (second row 904, second row 914, secondrow 924), (third row 906, third row 916, third row 926), (fourth row908, fourth row 918, fourth row 928), (fifth row 910, fifth row 920,fifth row 930).

[0089]FIG. 9 also includes first through fifth columns 932, 934, 936,938, and 940. Each column 932, 934, 936, 938, 940 corresponds to an OFDMsymbol time in the time domain.

[0090] Shading is used in FIG. 9 to illustrate segments corresponding toan exemplary channel within the particular sector. For example, duringthe OFDM time interval represented by first column 932, a trafficchannel for sector 1 corresponds to and uses the 3 tone frequenciesrepresented by first row 902, second row 904, and third row 906. In theFIG. 9 example, the three sectors allocate tones to channels using thesame allocation scheme. Thus in sectors 2 and 3 the same tones are usedfor the channel as in sector 1.

[0091] As the OFDM symbol time changes through the superslot 943,data/control tone hopping occurs and the tone frequencies used by thedata/control channels change. It can be seen that for the data/controltraffic channel segment in a given sector, there is a correspondingdata/control traffic channel segment in each of the other 2 sectors,since each sector in the exemplary embodiment has the same configurationof frequency tones and time intervals. The segments in the 3 sectorswhich correspond to the same channel are sometimes referred to as“corresponding channel segments.”

[0092]FIG. 10 shows a frequency vs time graph 1000 to illustratemultiple corresponding data/control traffic channel segments in the 3sectors.

[0093] First through fifteenth rows 1002, 1004, 1006, 1008, 1010, 1012,1014, 1016, 1018, 1020, 1022, 1024, 1026, 1028, 1030 of FIG. 10correspond to the same frequency tones as rows 902, 904, 906, 908, 910,912, 914, 916, 918, 920, 922, 924, 926, 928, 930 of FIG. 9,respectively. First though fifth columns 1032, 1034, 1036, 1038, 1040 ofFIG. 10 correspond to the same OFDM symbol times of first through fifthcolumn 932, 934, 936, 938, and 940 of FIG. 9, respectively. A super slot1043 defined by boundary lines 1031 and 1041 of FIG. 10, corresponds tothe super slot 943 of FIG. 9.

[0094] The area with line shading descending from left to right is usedto indicate a first set of corresponding data/control traffic segments,e.g., segments which correspond to the same channel. The area with lineshading ascending from left to right represents a second correspondingdata/control traffic segment in FIG. 10. For example, in the OFDM timeinterval represented by second column 1034, the first data/controltraffic segment in sector 1 uses frequency tones represented by firstrow 1002, third row 1006, and sixth row 1010, while the seconddata/control traffic segment in sector 1 uses frequency tonesrepresented by second row 1004 and fourth row 1008.

[0095] In the exemplary implementation, it can be seen that for anycontrol or data traffic channel segment in a given sector, there is acorresponding control or data traffic channel segment in each of theother 2 sectors, which has the same configuration of frequency tones andtime intervals. Those segments in the 3 sectors are referred to as“corresponding channel segments” in the following discussion. Note thatbecause of the full synchronization between the sectors, inter-sectorinterference is concentrated between corresponding channel segments.Other channel segments normally see little or negligible inter-sectorinterference between each other.

[0096]FIG. 11 shows a frequency vs time graph 1100 to illustrate pilottone hopping sequences in the 3 sectors.

[0097] First through fifteenth rows 1102, 1104, 1106, 1108, 1110, 1112,1114, 1116, 1118, 1120, 1122, 1124, 1126, 1128, 1130 of FIG. 11correspond to the same frequency tones as rows 902, 904, 906, 908, 910,912, 914, 916, 918, 920, 922, 924, 926, 928, 930 of FIG. 9,respectively. First though fifth columns 1132, 1134, 1136, 1138, 1140 ofFIG. 11 correspond to the same OFDM symbol times of first through fifthcolumn 932, 934, 936, 938, and 940 of FIG. 9, respectively. A super slot1143 defined by boundary lines 1131 and 1141 of FIG. 11, corresponds tothe super slot 943 of FIG. 9.

[0098] The pilot tone hopping sequences are indicated by horizontal lineshading in FIG. 11. Not all the pilot tone hopping sequences used ineach individual sector of a cell are the same to facilitate, among otherthings, sector identification of a mobile node. Thus, in FIG. 11 thepilot tone hopping sequences are shown to be different in each sector ofthe three sector cell. FIG. 11 illustrates the pilots by horizontalshading in the 3 sectors in a cell where no pilots overlap.

[0099] In accordance with the invention, the pilots used in each of theexemplary cell's sectors have the same value of SLOPE, but differentsets of offsets {O_(j)}. These known offsets may be included in thepilot value information 562 stored in the base station and/or the mobilenode pilot value offset information 648. In the example, in the 3-sectorcell, sector 1 uses offsets {O_(j,1)}, sector 2 uses offsets {O_(j,2)},and sector 3 uses offsets {O_(j,3)}. The offset sets {O_(j,1)},{O_(j,2)}, and {O_(j,3)} are not identical resulting in differentfrequencies being used for pilots in different sectors at the same time.In one embodiment, the offset sets are completely non-overlapping, thatis, no two elements in the offset sets are identical. Hence, the pilotsin different sectors do not interfere with each other. In anotherembodiment, {O_(j,2)} and {O_(j,3)} are derived from {O_(j,1)}:O_(j,2)=O_(j,1)+D₂ mod N, and O_(j,3)=O_(j,1)+D₃ mod N, for all j, whereD₂ and D₃ are two non-zero constants determined by the sector indices.

[0100] In accordance with the invention, the pilot hopping sequences anddata hopping sequences multiplex. That is, at a given OFDM symbol time,if one pilot sequence occupies the same tone as another data sequence,then the tone is used by the pilot sequence to the exclusion of the datathat would have been transmitted on the tone. Effectively, the datasequence is punctured at that OFDM symbol time. The punctured, e.g.,omitted, data may be recovered from the transmitted data using errorcorrection codes and error correction techniques.

[0101]FIG. 12 shows a frequency vs time graph 1200, which is acombination or overlay of FIGS. 10 and 11 and is used to illustrate thedata/control sequences of FIG. 10 being punctured by the pilot sequenceof FIG. 11. Each row corresponds to one frequency with each horizontalsection corresponding to a different symbol transmission time.

[0102] First through fifteenth rows 1202, 1204, 1206, 1208, 1210, 1212,1214, 1216, 1218, 1220, 1222, 1224, 1226, 1228, 1230 of FIG. 12correspond to the same frequency tones as rows 902, 904, 908, 910, 916,918, 920, 922, 924, 926, 928, 930 of FIG. 9, respectively. First thoughfifth columns 1232, 1234, 1236, 1238, 1240 of FIG. 12 correspond to thesame OFDM symbol times of first through fifth column 932, 934, 936, 938,and 940 of FIG. 9, respectively. A super slot 1243 defined by boundarylines 1231 and 1241 of FIG. 12, corresponds to the super slot 943 ofFIG. 9.

[0103] Line shading descending from left to right is used to indicatesegments corresponding to a first data or control channel. Line shadingascending from left to right indictes segments corresponding to a seconddata or control corresponding channel. Circles on top of thedata/control channel segments represent pilot tones punching through thedata/control sequences to the exclusion of the data which would havebeen transmitted in the segment.

[0104] When the sectorized OFDM spread spectrum system is used in acellular network, in accordance with the invention, neighboring cellsuse different values of SLOPE to determine the pilot and data tonehopping sequences. In the exemplary system of the invention, the offsetsets {O_(j,1)}, {O_(j,2)}, and {O_(j,3)} are the same in each of thesystem's numerous cells. Different cells need not, and often are not,synchronized in terms of tone frequencies, OFDM symbol timing, tonehopping sequences, channel segments or super slot boundaries even thoughwithin an individual cell sectors may have such features/characteristicsin common.

[0105] Power allocation across sectors of a cell and within a sector inaccordance with various features of the invention will now be described.

[0106] The fact that inter-sector interference mainly occurs betweencorresponding channel segments imposes a constraint on the powerallocation across corresponding channel segments in the sectors of acell.

[0107] For the sake of description, first suppose that correspondingchannel segments are all active, i.e., being used to transmit signals.In accordance with a feature of the invention, the transmission powerallocated to corresponding channel segments are substantially the samein each sector of a cell. For example, in the 3-sector system, if all 3corresponding channel segments are active, then the difference betweenthe transmission powers for those channel segments in the 3 sectorsshall be no more than a parameter, Delta. The scheduler 534 of FIG. 5,in the exemplary embodiment, is responsible for coordinating the powerallocation in each of the cell's sectors in a centralized manner.

[0108] The value of Delta, which may be stored in the base station asDelta information 564, affects the potential impact due to theinter-sector interference. For example, for a large Delta, thetransmission powers of two corresponding channel segments may be quitedifferent. Consequently, the inter-sector interference may cause largeinterference on one of the two corresponding channel segments that hassmaller transmission power. In one embodiment of the invention, Delta564 is set to be a constant, for example, zero. In another embodiment ofthe invention, Delta 564 may vary. Indeed, in accordance with theinvention, the value of Delta 564 may be different from one group ofcorresponding channel segments to another. For example, Delta forcorresponding control channel segments may be, and sometimes is,different from that for corresponding data traffic channel segmentsreflecting, from a policy perspective, tolerance for different levels ofinterference on different channels. In one embodiment of the invention,Delta is a function of burst data rates used in corresponding channelsegments. For example, consider corresponding traffic channel segments.If one of the segments uses high channel coding and modulation rate, forexample to support high burst data rate, a small value of Delta isdesirable and, in accordance with the invention, used. As part of itsfunction, the scheduler 534 determines the appropriate value of Delta564 when the scheduler 534 coordinates the power allocation and burstdata rate allocation in the sectors of a cell.

[0109] In accordance with the invention, the scheduler 534, includingroutine 542 of FIG. 5, can independently pick wireless terminals to bescheduled in corresponding data traffic channel segments of the cell'ssectors. The achieved burst data rates depend on the power allocationdetermined by routine 540 of FIG. 5 and the channel condition of thescheduled wireless terminals, e.g., as indicated by information 568, andthus may be different in different sectors of a cell.

[0110] The constraint on the power allocation across correspondingchannel segments in the cell's sectors does not impose a similarconstraint on the power allocation across different channel segmentswithin a sector. Indeed, in a given sector, different channel segmentsmay be allocated quite different amount of transmission power. Forexample, consider corresponding traffic channel segments. Suppose thereare two traffic channel segments at a given time. The scheduler 534 mayassign via routine 542 of FIG. 5 a wireless terminal of bad channelcondition to the first traffic channel segment in each of the sectors,and assign a wireless terminal of good channel condition to the secondtraffic channel segment in each of the sectors. Then, the scheduler 534can optimally balance the power allocation in the two traffic channelsegments. For example, the scheduler 534 allocates via routine 540 alarge portion, e.g., 80% or more, of transmission power to the firsttraffic channel segments to gain service robustness for the bad channelwireless terminals, and a small portion, e.g., 20% or less, oftransmission power to the second traffic channel segments to achievehigh burst data rate. In accordance with the invention, the dynamicrange of the allocated power between the two traffic channel segments inthe same sector may be large, e.g., greater than 3 dB relative powerdifference while the dynamic range of the allocated power acrosscorresponding traffic channel segments in the cell's sectors is limited,e.g., less than 3 dB relative power difference in some embodiments.

[0111]FIG. 13 illustrates the power allocation between traffic channelsegments in the same sector and across corresponding traffic channelsegments in multiple sectors of a cell for an exemplary case with twotraffic channel segments, and a value of Delta=0. In Table 1300 of FIG.13, first column 1308 lists the traffic segment number, second column1310 lists the sector 1 power allocation information, third column 1312lists the sector 2 power allocation information, and fourth column 1314lists the sector 3 power allocation information. First row 1302 of table1300 lists column header information. Second row 1304 lists trafficchannel 1 power allocation information across the three sectors. Thirdrow 1306 lists traffic channel 2 power allocation information across thethree sectors. In the example, Delta=0, i.e., the allocation tocorresponding channels in each sector of the cell is the same while thedifference in allocation of power between channels is large, e.g., adifference being a factor of 4.

[0112] Consider the following exemplary embodiment of the inventionincluding 2 adjacent sectors, including 2 channels in each sector, andwith base station transmit power control on each channel within eachsector of the cell in accordance with the invention. CELL SECTOR 1 (S1)SECTOR 2 (S2) CHANNEL 1 (C1) CHANNEL 1 (C1) SECTOR 1 POWER SECTOR 2POWER CHANNEL 1 (S1PC1) CHANNEL 1 (S2PC1) CHANNEL 2 (C2) CHANNEL 2 (C2)SECTOR 1 POWER SECTOR 1 POWER CHANNEL 2 (S1PC2) CHANNEL 2 (S1PC2)

[0113] The transmitter may be controlled to operate on a first andsecond communications channel in a synchronous manner with transmissionsmade into both first and second sectors.

[0114] In the exemplary case, the total transmission power of the tonescorresponding to the first channel in the first sector of the cell(S1PC1) is controlled to be greater than 20% and less than 500% of thetotal power of the tones transmitted in the second sector correspondingto the first channel (S2PC1) during a period of time, e.g., a subset ofsymbol times. This may be represented by a first channel wideinter-sector transmission power control range: 20%<(S1PC1/S2PC1)<500%.

[0115] In some embodiments, controlling the total transmission power ofthe tones corresponding to the first channel includes limiting the totalpower used in the first subset of symbol times to no more than a fixedfraction of a maximum average total transmission power used by thetransmitter in the first sector during any 1 hour period, the fixedfraction also being used to limit the total transmission power of thetones corresponding to the first channel in the second sector during thefirst subset of symbol times to be no more than the fixed fraction of amaximum average total transmission power used by the transmitter in thesecond sector during any 1 hour period, said fixed fraction being lessthan 100%.

[0116] In some embodiments, the total transmission power of the tonescorresponding to the first channel in the first sector of the cell(S1PC1) is controlled to be greater than 50% and less than 200% of thetotal power of the tones transmitted in the second sector correspondingto the first channel (S2PC1) during a period of time, e.g., anothersubset of symbol times. This can be represented by a first channelnarrow inter-sector transmission power control range:50%<(S1PC1/S2PC1)<200%. The base station may monitor the number ofsymbols in a constellation being used for an interval of time, and usethat information to decide whether to apply the wide inter-sectorchannel control range or the narrow inter-sector channel control range.With a larger number of symbols in a constellation, e.g., modulationwith more elements per set, the channel is more susceptible tointerference noise, and therefore, the narrower inter-sector powercontrol range is selected by the base station, allowing the base stationto more tightly control the levels of interference between users withinsectors, and keep that interference level to an acceptably low level.The base may also make decisions as to whether to use the wideinter-sector power control range or the narrow inter-sector powercontrol range based upon the channel coding rate, e.g., is the codingrate a slower coding rate or a faster coding rate. If a channel is usingthe faster coding rate for an interval of time, the base station shoulduse the narrower inter-sector transmission power control range, sincethe faster range will make the user, more susceptible to interference,and interference levels between users can be more tightly controlled andmanaged by the base station to maintain an acceptable level if thenarrower inter-sector transmission power control range is used.

[0117] In some embodiments, the interval or period of time, e.g., thesubsets of symbol times at which the transmission power control on aparticular channel concerning two adjacent sectors uses a tighterinter-sector power control range or a narrower inter-sector powercontrol range, includes at least 14 consecutive symbol times.

[0118] In some embodiments, the total transmission power of the tonescorresponding to the first channel in the first sector may be equal tothe total power of the transmitted tones in the first channel of thesecond sector during a period of time, e.g. interval of symbol times.This may be described as: S1PC1=S2PC1. FIG. 13 illustrates such a casewhere the power allocation to traffic segment 1=80% for both sector 1and sector 2 (second row 1304, column 2 1310 and columns 3 1312).

[0119] In some embodiments, within a given sector, e.g., the firstsector, the total power of the tones transmitted in the first sector forthe first channel (S1PC1) may be greater than 200% or less than 50% ofthe power of the tones transmitted in the first sector for a secondchannel (S1PC2) for a period of time. This inter-channel transmissionpower control range within a sector may be represented by:((S1PC1/S1PC2)<50%) or (S1PC1/S1PC2>200%). In the example of FIG. 13such an embodiment is shown, S1PC1=80% (second row 1304, second column1310) and S1PC2=20% (third row 1306, second column 1310);S1PC1/S1PC2=400%. This allows a wide range of power selections availableto the base station to match users to power levels.

[0120] The interval of time at which the base station controls thedifference in transmission power levels between the two channels withina given sector of a cell at greater than 200% or less than 50% may be ainterval of at least 14 consecutive symbol times.

[0121] In accordance with the invention, wireless terminals may beidentified as being in boundary areas, e.g., sector boundary areas. Theallocation of communication resources, e.g., channels, to wirelessterminals may be controlled. In accordance with the invention, thoseresources may include a channel that limits the base station's totaltransmission power of its tones controlled to be<10% total transmissionpower of the corresponding tones in the same channel in an adjacentsector to the boundary wireless terminal's sector. Thus, in such a caseratio of base station total transmission power on corresponding tonesfor the same channel between adjacent sectors would be 10% or less forone sector and 1000% or more for the adjacent sector. In otherembodiments, the <10% level may be 0%; effectively no power transmissionon same channel in the adjacent boundary sector. These implementationwith a channel in one sector allocated little or no power, in accordancewith the invention, is useful for operation of wireless terminals insector boundary regions where high levels of interference would normallybe experienced, e.g. regions 315, 317, and 319 of FIG. 3.

[0122] The identification and classification of wireless terminals 600of FIG. 6 to be in boundary areas, e.g., sector boundary regions, andthe allocation or resources based upon the identification may beperformed by the base station under the control routines 528 includingthe inter-sector interference routine 536 of FIG. 5, wirelessterminal/traffic & segment matching routine 542 of FIG. 5 and powerallocation routine 540. The identification of a wireless terminal 600 ina boundary area may be made based upon feedback information obtainedfrom the wireless terminal 600 that the base station 500 receives andprocesses; the feedback information may include experienced levels ofinter-sector interference, background interference and locationinterference. The wireless terminal 600 may collect MN channel conditioninfo 632 and report such information to the base station 500 under thedirection of the channel condition monitoring/reporting routine 628; theinformation will be available to the base station 500 in the MN channelcondition information 568.

[0123] Next, consider that corresponding channel segments need not allbe active. Note that an inactive segment does not cause inter-sectorinterference to other corresponding channel segments and is also notaffected by the inter-sector interference from other correspondingchannel segments. Therefore, in accordance with one embodiment of theinvention, when the scheduler 534 coordinates the power allocation in acell's sectors, only the active segments are taken into account.

[0124] If a wireless terminal, e.g., MN 600 of FIG. 6, is located at asector boundary, e.g., region 315, 317, or 319 of FIG. 3, it mayexperience a significant amount of inter-sector interference. In oneembodiment of the invention, the scheduler 534 uses inter-sectorinterference routine 536 and matching routine 542, to assign segments ofa first traffic channel to a wireless terminal in a sector boundary andthe corresponding traffic channel segments to wireless terminals innon-sector boundary areas in the other sectors. In another embodiment ofthe invention, the scheduler 534 via routines 538 and 542 assigns onetraffic channel segment to a sector boundary wireless terminal, andkeeps one or more corresponding traffic channel segments inactive in theother sectors, so as to reduce the inter-sector interference. In such acase, frequencies assigned to the wireless terminal in the sectorboundary area will not be subjected to interference from adjacentsectors since the tones are left unused in those sectors. In oneembodiment, there is a pattern of utilizing a given traffic channelsegment, in which a sector periodically keeps the segment inactive whilesome of the other sectors keep the segment active. The pattern can befixed such that the sectors do not have to coordinate each other in areal time fashion. For example, one sector (sector A) keeps a trafficsegment inactive (i.e., not assign it to any wireless terminal in thesector), while the other two sectors (sectors B and C) assign thesegments to the wireless terminals in the sector boundaries between Aand B and between A and C. In the subsequent traffic segment, sector Bkeeps a traffic segment inactive while the other two sectors assign thesegments to the wireless terminals in the sector boundaries between Band A and between B and C. Then, in the subsequent traffic segment,sector C keeps a traffic segment inactive while the other two sectorsassign the segments to the wireless terminals in the sector boundariesbetween C and A and between C and B. The whole pattern then repeats,without explicit and real time coordination among the three sectors.

[0125] One consequence of full timing and frequency synchronizationacross sectors of a cell is that it may be difficult for a wirelessterminal, e.g. MN 600 of FIG. 6, especially close to the sectorboundary, e.g., boundary 446 or 446′ of FIG. 4, to figure out whichsector 654 of FIG. 6, a received channel segment has come from. In orderto differentiate the channel segments across the sectors, distinctscrambling bit sequences may be used in different sectors.

[0126] Scrambling is a well-known method to randomize the transmittedsignal. There are a number of ways to implement scrambling. Considerbelow a particular implementation for illustration. It is understoodthat the principles of the invention do not rely on the particularexemplary implementation. At the transmitter 514 of FIG. 5, at a givenOFDM symbol transmission time, symbols from various channel segments,generated by the encoders of individual channel segments, aremultiplexed to form a symbol vector, which is then used to generate theOFDM symbol signal to be transmitted. The scrambling bit sequence is arandom binary sequence, known to both the transmitter 514 and thereceiver 610 of FIG. 6. The symbol vectors are phase-rotated in theexemplary embodiment based on the scrambling bit sequence. At thereceiver 610, the same scrambling bit sequence is used to de-rotate thereceived symbol before decoding takes place.

[0127] In accordance with one embodiment of the invention, distinctscrambling bit sequences are used in different sectors and thesector/scrambling information is stored in the mobiles. The basestation, 500 of FIG. 5, uses different scrambling bit sequences in the 3sectors to generate their respective transmit signals. The wirelessterminal receiver 610 of FIG. 6 uses the particular scrambling bitsequence, corresponding to the sector in which it is located, toselectively demodulate the signal from an intended sector transmissionof the base station 500. Alternatively, the wireless terminal receiver610 may use multiple scrambling bit sequences to demodulate the signalsfrom multiple sector transmissions of a base station 500 or frommultiple base stations simultaneously with the scrambling sequence usedcorresponding to the one used by the sector which transmitted the signalbeing recovered.

[0128] Channel condition measurement and reporting features of theinvention will now be described. In order to facilitate the schedulingfor downlink traffic channel segments, such as power allocation andburst data rate allocation, a wireless terminal 600 of FIG. 6 maymeasure its downlink channel condition under control of routine 628 ofFIG. 6 and periodically send the channel condition report includingdata/info 632/634 of FIG. 6 to the base station 500 of FIG. 5.

[0129] The channel condition of a wireless terminal 600 may be in twocharacteristic regions. For the sake of description, assume that thechannel condition is measured in terms of SIR (Signal InterferenceRatio). In the first region, e.g., the non-sector boundary region, theSIR is limited by the inter-cell interference or the wirelesspropagation loss, while the inter-sector interference is a smallcomponent. In that case, the base station 500 can increase the receivedSIR of a traffic channel segment to the wireless terminal 600 byallocating high transmission power under control of routines 538 and 540of FIG. 5. In the second region, e.g., the inter-sector boundary region,the SIR is mainly limited by the inter-sector interference. In thatcase, given the constraint on power allocation, e.g., a small Deltabetween sectors across corresponding data traffic channel segments inthe cell's sectors, allocating high transmission power does notremarkably increase the received SIR since the power of the interferenceincreases as the power is increased. The above two regions represent theextreme channel condition characteristics. In reality, the channelcondition of the wireless terminal 600 may more typically be in-betweenthe two extreme regions which were just described.

[0130] In accordance with the invention, the wireless terminal 600estimates, e.g., measures its channel condition characteristics undercontrol of routine 628 and notifies the base station 500 of thedetermined channel information. This allows the base station 500 to makesensible scheduling decisions in terms of power and burst data rateallocation. In one embodiment of the invention, data 632,634 shown FIG.6 is included in a downlink channel condition report sent to the basestation. In some implementations, the wireless terminal 600differentiates the SIR due to inter-sector interference via routine 536of FIG. 5 and SIR due to other types of impairments such as inter-cellinterference via routine 536 of FIG. 5 and provides such information tothe base station. This allows the base station to perform powerallocation decisions based on inter-sector feedback information and notsimply a single interference indicator which makes it difficult todetermine if allocating more power will have a desired beneficialresult.

[0131] Use of a relatively high power tone or tones, referred to here asa beacon signal, will now be described. To facilitate various downlinkoperations, in accordance with the invention, the base station 500 ofFIG. 5 may frequently and/or periodically transmit a beacon signal undercontrol of signal generation routine 532 as a function of information530 which includes beacon info 548. Each beacon signal is an OFDM signaltransmitted over, e.g., during one single symbol transmission period.When a beacon signal is transmitted, most of the transmission power isconcentrated on a small number of tones, e.g., one or two tones whichcomprise the beacon signal. Many or most of the tones which are not usedfor the beacon signal may, and often are, left unused. The tones whichform the beacon may include 80% or more of a maximum average total basestation power used by said base station to transmit in a sector during abeacon signal transmission time, which may, e.g., in some embodiments bea symbol time. In some embodiments, some additional tones, may carrysignal at the same time as the beacon transmission, and the total powerlevel for those tones is less than or equal to 20% of the maximumaverage base station power used by the base station to transmit in thesector at the time of beacon transmission.

[0132] The graph 1400 of FIG. 14 shows an ordinary OFDM signal. Thevertical axis 1402 represents the power allocated to tones while thehorizontal axis 1404 corresponds to tone frequency. Individual bars1406, 1408, 1410, 1412, 1414, 1416, 1418, 1420, 1422, 1424 eachcorrespond to the level of power for each of the distinct exemplary OFDMfrequency tones at some instant of time, e.g., the symbol period. It maybe seen that the total power is broken up relatively uniformly betweenthe various frequency tones.

[0133] The graph 1500 of FIG. 15 shows an exemplary beacon signal inaccordance with one exemplary embodiment of the present invention. Thebeacon signal includes two tones 1506, 1508. The majority of the sectortransmission power is allocated between the two tones 1506, 1508 of thebeacon each of which is allocated approximately 45-50% of the totalpower. The vertical axis 1502 represents per tone power while thehorizontal axis 1504 corresponds to tone frequency. In the FIG. 15example, this results in two tones having approximately the same totalpower as the tones normally used to transmit data. Individual bars 1506,1508 correspond to the level of power for each of two selected OFDMfrequency tones at the instant of time of beacon transmission. It may beseen that the total power is concentrated on the two selectedfrequencies at the time of beacon transmission. The significantconcentration of sector transmission power in a very limited number oftones differs significantly from conventional pilot tones where thepilots may be transmitted at power levels slightly higher than tonesused to transmit data.

[0134] The graph 1600 of FIG. 16 shows an exemplary beacon signal inaccordance with another embodiment of the present invention where thetotal power is allocated primarily to only one single frequency tonewhich is allocated approximately 90-100% of the total sectortransmission power. The vertical axis 1602 represents per tone powerwhile the horizontal axis 1604 represents frequency tone. A single bar1606 corresponds to the level of power for the single selected OFDMfrequency tone used as the beacon signal. It may be seen that the totalpower is concentrated on the one single frequency tone at the time ofbeacon transmission resulting in a beacon tone having a power level atleast 5 times that of the highest power tone used to transmit data inthe sector at other times.

[0135] One advantage of this concentration of power in a beacon signal,is the easy and rapid identification of the beacon signal(s) by themobile nodes, e.g. MN 600 of FIG. 6. This allows for the rapid and/oraccurate conveyance of information to the mobiles at the point of time abeacon is transmitted, e.g., super slot boundary synchronizationinformation, slope (cell) information, or sector information. Given thehigh power of the beacon tones, they are easy to detect with theprobability of a data tone being misinterpreted as a beacon tone beingrelatively low due to the normally large power difference between thebeacon tones and data tones.

[0136] In one embodiment of the invention, the beacon signal may betransmitted at a fixed OFDM symbol duration, for example, the first orthe last OFDM symbol, of a super slot. In this way, a beacon tone can beused to signal superslot boundaries. The beacon signal may repeat everysuper slot or every few super slots. The beacon signal is easy todetect, as it has extremely high power concentrated on just a few tones.Therefore, once the time position of the beacon signal has been located,the super slot boundaries can be promptly determined with a high degreeof certainty.

[0137] In another embodiment of the invention, the high power tone ortones used as a beacon signal is selected from a predefined group ofbeacon tones or tone sets. Tone sets are used where multiple high powertones form a beacon signal may vary with time. The sets of predefinedbeacon tones may be included as part of the stored beacon information548 included in the base station of FIG. 5 and the stored beaconinformation 652 of the wireless terminal. Using different beacon tonesets as the beacon signal can be used to indicate or convey certainsystem information including sector identification information. Forexample, the beacon signal may use 4 tones, as shown in FIG. 17. In thegraph 1700 of FIG. 17, the vertical axis 1702 represents per tone power,while the horizontal axis 1704 represents frequency. FIG. 17 shows a setof four beacon tones: B1 1706, A1 1708, A2 1710, and B2 1712. The pertone power for each of the beacons 1706, 1708, 1710, 1712 isapproximately the same with each beacon tone being allocatedapproximately 25% of the sectors total transmission power. The frequencylocation of various beacon tones, e.g., the two inner tones A1 1708 andA2 1710 is used to indicate the value of SLOPE used in the cell. Thefrequency location of some tones, e.g., the two outer tones B1 1706 andB2 1712 is used to indicate the boundary of the frequency band used inthe cell for transmission purposes and/or optionally the sector index.Beacon signals of neighboring cells will have different inner beacontone frequency location A1 1708 and A2 1710 to indicate different slopevalues. Thus in a given cell, the beacon signals of different sectorsmay have different B1 1706 and B2 1712 tone locations. Assuming that theouter beacon tones B1 1706 and B2 1706 are used to indicate frequencyboundaries, these may be the same in each sector of a cell assuming theuse of the same frequency band in each sector.

[0138] The time at which particular beacon signals are transmitted canbe used to indicate more than just slot boundaries. FIG. 18 shows agraph 1800 of frequency vs OFDM symbol time illustrating differentpossible types of beacons being transmitted in the time domain inaccordance with various possible embodiments of the invention. Thevertical axis 1802 represents frequency and the horizontal axis 1804corresponds to OFDM symbol time. Different beacon signals will bedescribed as corresponding to a particular beacon type based on theinformation it conveys alone or in combination with other beaconsignals.

[0139] A type 1 beacon signal 1806 is shown to be transmitted at thestart of a super slot. After a time interval of k super slots 1812,where k is an integer value, a type 2 beacon 1808 is transmitted. Then ksuper slots 1814 later, a type 3 beacon 1810 is transmitted. The tonefrequencies and/or beacon tone power levels for each of the threebeacons 1806, 1808, 1810 are different. The type 1 beacon 1802 may beused to convey frequency floor information indicating a lower frequencyboundary of frequency band being used in a sector. The type 2 beacon maybe used to provide an index to slope, e.g., slope indicator, from whicha wireless terminal can determine the cell's slope. Using the type 2beacon to determine slope allows a wireless terminal to determine whichcell the mobile node is located in. A type 3 beacon 1810 is used toconvey sector information (e.g. allow the mobile to identify the sectorlocation 1, 2, 3) via e.g. an index table of sector numbers or pilotoffsets corresponding to specific frequency tone values in the samemanner a type 2 beacon can be used to convey cell information, e.g.,slope information. As discussed above, different base stations may bepre-configured with different values of slope, and different values forpilot offsets in different sectors, which are used to control thehopping sequences within a base station's cell.

[0140]FIG. 19 shows a graph 1900 of frequency vs OFDM symbol timeillustrating the concept of transmitting alternating beacons types inthe time domain in accordance with one embodiment of the presentinvention to convey information. The vertical axis 1902 representsfrequency while the horizontal axis 1904 represents OFDM symbol time. Inthe example shown in FIG. 19, the base station 500 of FIG. 5 transmitsalternating beacon types in the following sequence: type 1 beacon 1906,type 2 beacon 1908, type 1 beacon 1910, type 2 beacon 1912, type 1beacon 1914, type 2 beacon 1916, type 1 beacon 1918, type 2 beacon 1920.All of the type 1 beacons 1906, 1910, 1914, 1918 are transmitted at thesame frequency tone f₁ 1922. Type 2 beacons 1908 and 1916 aretransmitted at frequency tone f_(2a) 1924 while type 2 beacons 1912 and1920 are transmitted at frequency tone f_(2b) 1926. In the time domainthe type 2 beacons switch between the two frequency tones, f_(2a) 1924and f_(2b) 1926, alternately. The mobile node 600 of FIG. 6 can identifythe type one beacons based on beacon tone frequency. The mobile node 600may be able to process the two distinct type two beacons via an indextable which converts each of the tone frequencies to an index number andultimately to one slope hopping value 646 of FIG. 6 specific to onespecific cell 656 of FIG. 6. The mobile node 600 will receive two indexnumbers, one of which will correspond to the slope index 650. The accessnode 500 will operate on a fixed number of slope index values with adefined slope indicator equation. Based the mobile's knowledge of thatdata, the mobile 600 can determine which index 650 corresponds to theslope 646.

[0141] As an example, consider that the slope index range is 0≧X_(S)≧79and that the slope indicator equation is (X_(S)+39) Mod 80. X_(S)represents the index to slope for the access node 500. The access node500 when it transmits the type 2 beacon, alternates between the tonefrequencies corresponding to X_(S) and (X_(S)+39) Mod 80. In anexemplary case with a value of slope index=50, the exemplary access nodetransmits type 2 beacons for index values: 50 and 9. The mobile node 600may receive the index 50 beacon followed by the index 9 beacon or theindex 9 beacon followed by the index 50 beacon, depending upon the timethat the mobile 600 first detected the type 2 beacon signal. In orderfor the mobile 600 to determine which is the X_(S) or slope index (firstbeacon), the mobile 600 uses the known information that the secondbeacon's index will be 39 index counts from the X_(S). If the mobile 600first receives 9 and then 50, the change in index counts is 41;therefore, the second received index value 50 is the real value to beused for slope index 650. If the mobile 600 first receives 50 and then9, the change in index counts is 39, therefore, the first received indexvalue 50 is the real value to be used for slope index 650.

[0142] By using an index to slope or slope indicator, diversity infrequency is provided allowing reconfiguration in case of failures on aspecific tone frequency.

[0143] The beacon may also be useful in identifying the cell and sectorlocation (656 and 654 of FIG. 6), and potentially more precise locationwithin the sector, of the mobile 600 receiving the beacon signal(s) andthus be useful to provide warnings of hand-offs and improve theefficiency in handoff operations. Also, by taking over some of thefunctions sometimes performed by the use of pilot hopping sequences andtransmitted pilot signals, such as synchronization to super slotboundaries, the number of pilots and/or pilot power can be reduced. Thusthe time of pilot data punch through may be reduced and there may alsobe a saving in power required to transmit and process pilots.

[0144] Various base station signaling, at different strength levels on aper tone basis and different repetition rates, of the present inventionwill be described and discussed, as used in an exemplary frequencydivision multiplexed communications system, e.g., an OFDM system. Foursignals shall be described, first signals which may include ordinaryOFDM signal as in FIG. 14, a second signal with high power levels, e.g.,a beacon signal as in FIG. 15, a third signal which include signalhaving ordinary OFDM signals power levels which may include, e.g, userdata, or if occurring concurrently with a beacon may have power levelsusing the power remaining after beacon allocation, and a fourth signal,e.g., another beacon signal as in FIG. 16 with high power levelscomparable with the second signal. The base station transmitter 514 ofFIG. 5 uses a set of N tones, e.g. included in tone info 550 of FIG. 5,where N is larger than 10, to communicate information using firstsignals over a first period of time at least two seconds long and insome embodiments the first period of time is at least 30 minutes. Thefirst signals may include, e.g., user data on traffic channels and maybe transmitted using data tone hopping sequences 554 of FIG. 5. A secondsignal, sometimes referred to as a beacon signal, may be transmittedduring a second period of time, where the beacon signal includes a setof X tones, included in tone info 550 where X is less than 5, and whereat least 80% of a maximum average total base station transmission powerused by the base station during any 1 second time period during thefirst period of time, is allocated to the set of X tones forming thebeacon signal. In some embodiments, the second period of time, used totransmit the second (beacon) signal, may be, e.g., the period of timeused to transmit an OFDM symbol 552 of FIG. 5. In some embodiments, thesecond period, e.g., beacon time period, repeats periodically during thefirst period. Some of the X tones (beacon) may be at predetermined fixedfrequencies; such fixed frequencies, (see FIG. 17), may be used toconvey information such as sector location. Some of the X tones (beacon)may have a fixed frequency offset≧0 from the lowest frequency tone inthe set of tones N; in this way the second signal (beacon signal) can beused to convey frequency boundary information to the wireless terminal600. Some of the X tones (beacon) may be transmitted at a frequencywhich is determined as a function of at least one of a base stationidentifier and a sector identifier. This may allow a wireless terminalto rapidly identify the cell and sector that it is operating in, quicklyobtain the data and pilot hopping sequences, and quickly synchronizewith the base station. In some embodiments, the number of X in thesecond (beacon) signal is one (see FIG. 16) or two (see FIG. 16). Thusthe base station's second (beacon) signal, transmitted with relativelyhigh power and with energy concentrated in one or a few tones, is easilydetectable by wireless terminals. In some embodiments, at least half ofthe N-X tones in the set of N tones but not in the set of X tones gounused during the period of the beacon transmission. In otherembodiments, none of the N-X tones in the set of N tones but not in theset of X tones are used during the beacon transmission time. Byrestricting transmission of non-X (beacon) tones during the secondsignal (beacon tone interval), the level of the second (beacon) signalcan be increased, and confusion with other signaling may be reduced,providing better detection and identification of the beacon signal bywireless terminals.

[0145] Third signal may also be transmitted over a third interval oftime. The third signal may include a set of Y tones included in tonefrequency info 550, where Y≦N, with each tone in third set of Y toneshaving 20% or less of said maximum average base station transmissionpower used by base station transmitter during any 1 second period duringthe first period of time. The third period of time may have the sameduration as the second period of time, e.g., occur concurrently with abeacon signal. In some embodiments at least two of data, control andpilot signals may be modulated on at least some of said set of Y tones.In some embodiments, the repetition rate of the set of Y (third signal)tones is at least 10 times the repetition rate of the set of X (secondor beacon signal) tones, while in other embodiments, the repetition rateof the set of Y (third signal) tones is at least 400 times therepetition rate of the set of X (second or beacon signal) tones.

[0146] A fourth signal may also be transmitted by the base station 500during a fourth period of time. The fourth signal includes G tonesincluded in tone frequency info 550 of FIG. 5, where G is less than 5and where at least 80% of the maximum average total base station powerused by the base station during any 1 second period during the firstperiod of time is allocated to the G tones. At least one of the G tonesis not in the set of X tones (second signal tone set) and the frequencyof at least one of the G tones is a function of at least one of a basestation identifier and a sector identifier. The fourth signal may alsorepeat periodically during the first time interval. The fourth signalmay be viewed as a second beacon signal being transmitted at a differenttime than the second signal and conveying different information.

[0147] Beacon signals, are structured, in accordance with the invention,to concentrate a relatively high level of power in a small number oftones. During the time of beacon transmission the non-beacon tones maycarry no information or in some instances, some of the non-beacon tonesmay carry signal but at a level significantly below the beacon tonelevels. The beacon tones by their characteristics are easy to detect andcan quickly convey information, e.g., cell and/or sector information,frequency boundary information, and/or synchronization information towireless terminals.

[0148] Uplink issues will now be described. In accordance with theinvention, the frequency, symbol timing, and super slot structures ofthe uplink signal generated by a wireless terminal may be slaved tothose of the downlink signal. Having full synchronization of thedownlink signal in each of the sectors, tone frequencies, OFDM symboltiming, and super slot boundaries synchronized to the uplink signal ineach of a cell's sectors will insure similar synchronization in theuplink where the uplink is slaved to the downlink.

[0149] In one preferred embodiment of the invention, the data tonehopping sequences and channel segments are synchronized across thesectors of a cell. In that case, inter-sector interference isconcentrated between corresponding channel segments.

[0150] In another embodiment of the invention, the data tone hoppingsequences are determined as a function of both the SLOPE parameter andsector index. In that case, there is no notion of corresponding channelsegments. A channel segment in one sector may interfere with multiplechannel segments in another sector of the same cell.

[0151] The present invention may be implemented in hardware and/orsoftware. For example, some aspects of the invention may be implementedas processor executed program instructions. Alternatively, or inaddition, some aspects of the present invention may be implemented asintegrated circuits, such as ASICs for example. Control means forcontrolling one or more transmitters may, and in various embodiments areimplemented as software modules of a control routine. The apparatus ofthe present invention are directed to software, hardware and/or acombination of software and hardware. Machine readable medium includinginstructions used to control a machine to implement one or more methodsteps in accordance with the invention are contemplated and to beconsidered within the scope of some embodiments of the invention.

What is claimed is:
 1. A method of operating a base station transmitterin a frequency division multiplexed communications system the basestation transmitter using a set of N tones to communicate informationover a first period of time using first signals, said first period oftime being at least two seconds long, where N is larger than 10, themethod comprising: transmitting during a second period of time a secondsignal including a set of X tones, where X is less than 5, and where atleast 80% of a maximum average total base station transmission powerused by said base station transmitter during any 1 second period duringsaid first period of time is allocated to said set of X tones.
 2. Themethod of claim 1, wherein said first period of time is at least 30minutes.
 3. The method of claim 1, where X is equal one or two.
 4. Themethod of claim 1, wherein at least half of the N-X tones which are insaid set of N tones but not in said set of X tones go unused during saidsecond period of time.
 5. The method of claim 4, wherein none of the N-Xtones in said set of N tones but not in said set of X tones are usedduring said second period of time.
 6. The method of claim 4, whereinmultiple ones of the N-X tones in said set of N tones but not in saidset of X tones are used during said second period of time.
 7. The methodof claim 1, wherein said communications system is an orthogonalfrequency division multiplexed system and wherein said second period oftime is a period of time used to transmit an orthogonal frequencydivision multiplexed symbol.
 8. The method of claim 7, wherein saidsecond period of time periodically repeats during said first period oftime.
 9. The method of claim 7, wherein said method further comprises:transmitting during a third period of time a third signal including aset of Y tones, where Y≦N, each tone in said third set of Y tones having20% or less of said maximum average total base station transmissionpower used by said base station transmitter during any 1 second periodduring said first period of time, said third period of time having thesame duration as said second period of time.
 10. The method of claim 9,wherein said third period of time and said second period of timeoverlap, the method further comprising: modulating at least two of data,control and pilot signals on at least some of said set of Y tones. 11.The method of claim 9, wherein said third period of time and said secondperiod of time are disjoint, the method further comprising: modulatingat least two of data, control and pilot signals on at least some of saidset of Y tones.
 12. The method of claim 7, wherein at least one of saidX tones is transmitted at predetermined fixed frequency; and whereinsaid at least one of said X tones is transmitted using a frequencyhaving a fixed frequency offset≧0 from the lowest frequency tone in saidset of N tones.
 13. The method of claim 7, wherein at least one of saidX tones is transmitted at a frequency which is determined as a functionof at least one of a base station identifier and a sector identifier.14. The method of claim 9, wherein for each repetition of said secondperiod of time in said first period of time there are at least Zrepetitions of said third period of time in said first period of timewhere Z is at least
 10. 15. The method of claim 14, wherein Z is atleast
 400. 16. The method of claim 7, further comprising: transmittingduring a fourth period of time a fourth signal including G tones, whereG is less than 5, and where at least 80% of said maximum average totalbase station transmitter power used by said base station transmitterduring any 1 second period during said first period of time is allocatedto said G tones.
 17. The method of claim 16, wherein the frequency of atleast one of said G tones is a function of at least one of a basestation identifier and a sector identifier, and wherein said at leastone of said G tones is not one of said set of X tones.
 18. The method ofclaim 17, wherein said second and fourth periods of time periodicallyrepeat during said first period of time.
 19. A base station for use in afrequency division multiplexed communications system the base station,the base station comprising: a transmitter that uses a set of N tones tocommunicate information; first control means coupled to saidtransmitter, for controlling the transmitter to transmit over a firstperiod of time using first signals, said first period of time being atleast two seconds long, where N is larger than 10; and second controlmeans coupled to said transmitter for controlling the transmitter totransmit during a second period of time a second signal including a setof X tones, where X is less than 5, and where at least 80% of a maximumaverage total base station transmission power used by said base stationtransmitter during any 1 second period during said first period of timeis allocated to said set of X tones.
 20. The base station of claim 19,wherein said first period of time is at least 30 minutes.
 21. The basestation of claim 19, wherein the first and second control means aredifferent portion of a control routine; and where X is equal one or two.22. The base station of claim 19, wherein at least half of the N-X toneswhich are in said set of N tones but not in said set of X tones gounused during said second period of time.
 23. The base station of claim22, wherein none of the N-X tones in said set of N tones but not in saidset of X tones are used during said second period of time.
 24. The basestation of claim 22, wherein multiple ones of the N-X tones in said setof N tones but not in said set of X tones are used during said secondperiod of time.
 25. The base station of claim 19, wherein saidcommunications system is an orthogonal frequency division multiplexedsystem and wherein said second period of time is a period of time usedto transmit an orthogonal frequency division multiplexed symbol.
 26. Thebase station of claim 25, wherein said second period of timeperiodically repeats during said first period of time.
 27. The basestation of claim 25, further comprising: third control means forcontrolling the transmitter to transmit during a third period of time athird signal including a set of Y tones, where Y≦N, each tone in saidthird set of Y tones having 20% or less of said maximum average totalbase station transmission power used by said base station transmitterduring any 1 second period during said first period of time, said thirdperiod of time having the same duration as said second period of time.28. The method of claim 27, wherein said third period of time and saidsecond period of time are disjoint, the method further comprising:modulating at least two of data, control and pilot signals on at leastsome of said set of Y tones.
 29. The method of claim 25, wherein atleast one of said X tones is transmitted at predetermined fixedfrequency.
 30. The method of claim 25, further comprising: fourthcontrol means for controlling the transmitter to transmit during afourth period of time a fourth signal including G tones, where G is lessthan 5, and where at least 80% of said maximum average total basestation transmitter power used by said base station transmitter duringany 1 second period during said first period of time is allocated tosaid G tones.
 31. The method of claim 30, wherein the frequency of atleast one of said G tones is a function of at least one of a basestation identifier and a sector identifier, and wherein said at leastone of said G tones is not one of said set of X tones.
 32. The method ofclaim 31, wherein said second and fourth periods of time periodicallyrepeat during said first period of time.